Two U.S. F/A-18 fighter jets dropped 500-pound, laser-guided bombs on Friday on an Islamic State mobile artillery piece used to shell Kurdish forces defending Arbil, the Pentagon said, and strikes later in the day involved a drone and four F/A-18s.

The United States has a consulate and, since Iraq's latest security crisis erupted in June, a joint military operations center staffed by 40 U.S. servicemen in Arbil, capital of the semi-autonomous Iraqi Kurdistan region.

Members of the U.S. Congress supported the air strikes, but Republicans demanded the president spell out a long-term strategy. Even Obama's closest Democratic allies made clear they wanted him to work with legislators, not circumvent them.

John Boehner, the Republican Speaker of the House of Representatives, said he was "dismayed" at the lack of long-range planning.

"The president needs a long-term strategy - one that defines success as completing our mission, not keeping political promises - and he needs to build the public and congressional support to sustain it," he said in a statement.

There was uncertainty, even expressed privately by some U.S. officials, about Obama's strategy, which he has said is not aimed at a sustained campaign against the Islamist militants who are threatening Iraqi government and Kurdish positions.

Friday's strikes were the first aggressive U.S. military action in Iraq 2-1/2 years after Obama withdrew the last American troops, fulfilling a promise he campaigned on to win office in 2008 and ending a bloody U.S. war that began in 2003.

Obama authorized air strikes late on Thursday to avert "a potential act of genocide" of tens of thousands of members of the ancient Yazidi sect who have taken refuge on a desert mountaintop from Islamic State forces. The United States has also begun dropping relief supplies to the refugees.

The White House said the military engagement would not involve ground forces. But reflecting Washington's pressure on Iraqi politicians to form a government that includes Sunnis, Shi'ites and Kurds, the White House said authorization for limited action could eventually include more military support to Iraqi security forces once the country forms a new "inclusive" government.

White House spokesman Josh Earnest said the initial U.S. support would be military strikes to protect American personnel in Iraq and to address an urgent humanitarian situation.

Obama discussed the crisis on Friday in a phone call with King Abdullah of Jordan, a close U.S. ally who has already seen IS fighters on his country's border with Iraq.

STRIKE TARGETED MILITANT ARTILLERY

Kirby said the decision to strike was made by the U.S. Central Command commander under the authorization from Obama.

He said the first strike occurred at 6:45 a.m. EDT, or 1:45 p.m. in Arbil (1045 GMT). According to military officials, it was launched from an aircraft carrier, the USS George H.W. Bush.

Later attacks included a drone strike on a mortar position and an attack by four F/A-18 jets on a convoy and a mortar position, the Pentagon said.

U.S. and European security officials said IS convoys had been heading toward Arbil. Western governments were concerned that one of the convoys might have been on its way to attack Christian communities in or near Arbil, one official said.

An allied government source familiar with intelligence reports said the main target of the first U.S. air strike was an Islamic State artillery battery comprised of U.S.-made weapons stolen from the retreating Iraqi army.

Washington has indications the artillery targeted in the strikes was destroyed, a U.S. defense official said.

Sunni fighters from the Islamic State, an al Qaeda offshoot bent on establishing a caliphate, have swept through northern Iraq since June. They are now encroaching on Arbil, seat of the Kurdish region's parliament and temporary home to scores of refugees who have fled other parts of Iraq.

While Obama has insisted that the United States will not commit ground troops again, since June he has ordered some 700 soldiers into Iraq to protect diplomatic personnel and facilities and to assess the state of Iraq's military, much of which melted away in the face of the Islamic State advance.